humidity compensation for a wooden pendulum

The tempo of a pendulum depends on its length. Shorter is faster.

All my clocks have wooden pendulums. It's the traditional material and has the advantage that, unlike steel pendulums, they do not expand much with heat. However wooden pendulums are very sensitive to changes in humidity and they swell and slow down when it is wet. This page describes an attempt to compensate for humidity.

4 June My experimental pendulum is pine-wood: 5 x 27 x 1000 mm. The pendulum weight is a 30cm length of 1 inch cold water pipe, 0.66kg, with the galvanisation machined off. It is regulated with a nut on a long 3mm threaded rod that positions the weight so the clock does one tick a second.
For this experiment I have added a block of wood between the weight and the regulation nut. In this block the grain of the wood is arranged so that the direction of maximum expansion - along the direction of the rings of the tree - runs more or less vertically.
The idea is that the wood block pushes the weight upwards when it is wet, hopefully the same amount as the pendulum lath expands downwards, along the grain.
The idea is so simple that I can't imagine that it is original.

Above: Under the pendulum weight is my block of expanding pieces of wood. It is 10cm long. I expect that 10cm will over-compensate for humidity. If so, I will shorten the block of wood until the compensation is correct.

5 June I adjusted the pendulum so that it is running about the same as a nearby uncompensated clock of similar construction.→

6 June Now both clocks are adjusted according to the DCF77 time signal from Frankfurt.
For fine adjustments like this the method is to add or remove small weights from a weight tray half way up the pendulum. For example 0.3 grams = 5 seconds a day for this pendulum. This avoids having to stop the clock to turn the regulation nut; the clock would take several hours to settle down again after being stopped and re-started.

14 June During the first week's run the 10cm wood block followed the humidity readings quite nicely and over-compensated, as expected. I cut the block down to 5cm - carefully saving the cut-off piece - and had readjusted the clock to DCF77 again by the morning of the 15th June.

22 JuneThe changes in rate are still related to humidity, not as wild as previously but far from even so I cut the block to half its previous size; it is now 2.5 cm long.

5 July There has been some improvement with smaller variations. The kinks in the clock and humidity graphs still change in the same direction so it is still over-compensating slightly. That means when the humidity increases the clock goes faster. So the block should be even shorter.

11 July I started recording again with a shorter wood block, see below.

19 July After an excellent start the pendulum became erratic. It seems that the wood block, currently 12mm high, is now too short. I will let it run a few more days and see what happens.

July 27 I added an 8 mm slice to the wood block so that it is now 20 mm high

August10 The two clocks are side by side on the same wall with a door between them. For the last week this door has been kept shut. Maybe this caused an improvement.

That concludes my experiments. There was a definite improvement and the results are good for a wooden pendulum. Another approach would be to put the clock, or at least the pendulum, into an air-tight case ... but I really prefer the way it looks now.

PS 25.11.21. I put a similar block of wood under the weight of my synchronome-like clock. It keeps time to within 1 second a day.

to a long pendulum clock

to a synchronome-like clock